A maleic acid-based copolymer having many carboxyl groups has so far been known to show superior chelating and dispersing effects, and is used in various fields with respect to detergent compositions, dispersing agents, cohering agents, scale inhibitors, chelating agents, fiber-treating agents, washing agents and the like.
Japanese Official Patent Gazette No. Heisei 3-2167 gives an example of use of a maleic acid-based copolymer as a pigment-dispersing agent, wherein the copolymer is produced by the steps of: charging a maleic acid component into a reaction vessel; beginning dropwise addition of acrylic acid and an aqueous hydrogen peroxide solution simultaneously; finishing the dropwise addition of these materials simultaneously; and thereby copolymerizing maleic acid and acrylic acid at a pH of 3.5 to 5.0. However, the maleic acid-based copolymer obtainable by this process contains a large amount of low molecular weight oligomer components. In addition, although a large amount of hydrogen peroxide is used for polymerization, a large amount of maleic acid remains in a reaction solution when the polymerization has finished. Moreover, hydrogen peroxide also remains in a large amount.
Also, Japanese Official Patent Provisional Publication No. Showa 62-218407 gives an example of use of a maleic acid-based copolymer as a dispersing agent and as a detergent composition. It is hard to say that the detergent composition described in this publication has washing power higher than conventional detergent compositions. The detergent composition described in the publication does not have desirable performance as a detergent in good balance. In the publication, a maleic acid-based copolymer is produced by the steps of: charging a maleic acid component into a reaction vessel; beginning dropwise addition of acrylic acid and an aqueous hydrogen peroxide solution simultaneously; finishing the dropwise addition of these materials simultaneously; and thereby copolymerizing maleic acid and acrylic acid while maintaining pH at about 4 to about 6. The obtained maleic acid-based copolymer contains a large amount of low molecular weight impurities. In addition, although a large amount of hydrogen peroxide is used for polymerization, a large amount of maleic acid remains in a reaction solution when the polymerization has finished. Moreover, hydrogen peroxide also remains in a large amount.
Furthermore, Japanese Official Patent Gazette No. Heisei 3-14046 shows that copolymers of ethylenically unsaturated mono- and dicarboxylic acids are used as outside film formation inhibitors for detergents and for washing agents. These copolymers are produced by the steps of: charging an ethylenically unsaturated dicarboxylic acid component into a reaction vessel; beginning dropwise addition of an ethylenically unsaturated monocarboxylic acid component and an aqueous hydrogen peroxide solution simultaneously; and finishing the dropwise addition of these materials simultaneously. However, the polymers obtained in this reaction are not copolymers which contain maleic acid at a high content and which have many carboxylic groups in their molecules. In addition, the polymers obtained contain a large amount of low molecular weight oligomers and deteriorate washability upon mud dirt when used as detergent compositions.
The present inventors studied about what physical properties of maleic acid-based copolymers are important for enhancing the washing power of detergent compositions containing the maleic acid-based copolymers. As a result, the inventors found that it is important to enhance calcium ion-capturability and clay-dispersibility of the maleic acid-based copolymers, and also found that it is important, to high level washing for the removal of mud dirt, that the maleic acid-based copolymers have both of a calcium ion-stabilization degree constant and a level of adsorption to clay particles within particular ranges. The inventors further found that since, when water having a high hardness is used, maleic acid-based copolymers having a high gelatability become insoluble and thereby greatly weaken their washing power, it is important to lower the gelatability of maleic acid-based copolymers.
Maleic acid-based copolymers obtained under conventional reaction conditions contain hydrogen peroxide at a high concentration and residual maleic acid in a large amount, and are insufficient with respect to the calcium ion-capturability, clay-dispersibility and gelatability (which are important to the enhancement of the washing power of detergent compositions). In addition, the calcium ion-stabilization degree constant, the level of adsorption to clay particles, and the gelatability are also unsatisfactory.